1. Field of the Invention
The present invention relates to a circuit for producing a clock signal for reproducing a PCM signal for use in record reproduction systems, such as VTRs or digital audio tape recorders of the rotary head type (R-DAT).
2. Prior Art
The PCM signal is reproduced using a clock signal (fch=9.408 MHz) having twice the frequency of minimum bit pulses thereof (hereinafter referred to as "PCM" signal frequency).
A PCM signal is recorded, for example, on a magnetic tape, which is thereafter helically scanned by a magnetic head mounted on a cylinder rotating at a high speed for reproducing the signal in various modes, for example, high-speed search, usual mode or trick reproduction, or for monitoring during fast forwarding or rewinding. FIG. 12 shows a PLL (phase locked loop) circuit which is used for reading the reproduced PCM signal.
When the speed of the magnetic head 1, relative to the magnetic tape 2, is different from that in the usual reproduction mode, the PCM signal from the head has a different frequency. Accordingly, the circuit is adapted to automatically vary the reproduction clock frequency fch (i.e. channel clock frequency) in accordance with the difference.
The PLL circuit has a phase comparator 3, low-pass filter 4, limiter circuit 5 and voltage-controlled oscillator (VCO) 6. The reproduced PCM signal from the head 1 and the reproduction clock signal from the VCO are negatively fed back to the phase comparator 3.
3. Problem to be Solved
For the conventional PLL circuit, the operating range of the VCO 6 is restricted to the neighborhood of the desired frequency by the limiter circuit 5 as represented by the hatched area of FIG. 13 so that the reproduction clock will not be locked at an improper frequency.
However, if the reproduced PCM signal frequency varies beyond a limit, there arises a need for the reproduction clock signal of a frequency outside the operating range of the VCO. The conventional circuit therefore has a problem of being unable to reproduce the PCM signal data in this case.
Unexamined Japanese Patent Publication No. SHO 62-8354 and "Radio Technology," April 1987, pp. 74-77 disclose a method for use in a special reproduction mode wherein the speed of rotation of the head cylinder is altered to use the reproduction clock of usual frequency. Nevertheless, this method involves the problem that the rotational speed of the cylinder motor must be so controlled as to maintain the head cylinder at a definite speed relative to the magnetic tape.